Optical switches, which can directly manipulate optical signals, are becoming increasingly important for optical networking. Accordingly, several techniques for switching optical signals have been developed. FIG. 1A shows a plan view of an optical switch 100 that uses some of the optical switching techniques described in U.S. Pat. No. 5,699,462 to Fouquet et al., entitled “Total Internal Reflection Optical Switches Employing Thermal Activation.” As illustrated in FIG. 1A and in the cross-sectional views of FIGS. 1B and 1C, optical switch 100 includes a planar lightwave circuit 110, a semiconductor substrate 120, a base plate 130, and a reservoir 140.
Planar lightwave circuit 110 is a plate of an optical material such as quartz containing crossing waveguide segments 112 and 114 and cavities 116 at the intersections of waveguide segments 112 with waveguide segments 114. Optical signals are generally input to optical switch 100 on one set of waveguide segments 112 or 114, and cavities 116 act as switching sites for the optical signals. In particular, a cavity 116 when filled with a liquid 142 having a refractive index matching the refractive index of the waveguides 112 and 114 transmits an optical signal from an input waveguide segment 112 or 114 into the next waveguide segment 112 or 114 along the same path. FIG. 1B shows a cavity 116A filled with liquid 142.
FIG. 1B also shows a cavity 116B that contains a bubble 146B that makes the switching site reflective. More specifically, total internal reflection at an interface 115 between an input waveguide 112 or 114 and bubble 146B reflects an optical signal and switches the optical signal into a crossing waveguide segment 114 or 112. Selectively creating a bubble in one of the cavities 116 along the initial path of an optical signal can make that cavity reflective and switch the optical signal onto the crossing waveguide segments 114 or 112 corresponding to the reflective cavity. If none of the cavities 116 along the path of an optical signal are reflective, the optical signal passes straight through optical switch 100.
Semiconductor substrate 120 contains electronic circuitry including heating elements 122 positioned in cavities 116. Selectively activating a heating element 122 vaporizes liquid in the corresponding cavity 116 and activates (i.e., makes reflective) the switching site corresponding to the cavity 116 containing the activated heating element 122. The activated heating element 122 then continues heating to keep the bubble stable and keep the switching site reflective. If the heating element 122 is turned off, bubble 146 and surrounding liquid 142 cool, causing bubble 146 to collapse and the cavity 116 to refill with liquid 142.
FIG. 1C illustrates the process of activating the switching site corresponding to cavity 116A. For activation, power is applied to a heating element 122A to raise a portion of liquid 142 to a temperature high enough to form a bubble 146A in cavity 116A. The required temperature corresponds to the nucleation energy for bubble formation and is generally well above the boiling point of liquid 142. Accordingly, bubble 146A expands rapidly even if the power supplied to heating element 122A decreases. Expanding bubble 146A pushes liquid 142 out of cavity 146A. Liquid from 116A eventually flows to a reservoir 140 via liquid layer 150, which underlies planar lightwave circuit 110, one or more holes 156 through substrate 120, and an inlet/outlet 154 through base plate 130. Additional channels 152 etched in planar lightwave circuit 110 can aid flow to holes 156.
A problem for optical switch 100 is the flow of liquid 142 from a cavity 116A being activated into neighboring cavities 116B containing established bubbles 146. The flow arises because existing bubbles 146B compress easily and are generally closer to the activating bubble 146A than is a gas cushion 144 in reservoir 140. Liquid 142 flowing into a cavity 116B containing an established bubble 146B can disrupt the reflection at that switching site and interfere with switching of an optical signal, creating crosstalk or noise in the optical signals.
In view of the need for clean and stable switching of optical signals without hydrodynamic crosstalk, structures and operating methods are sought that prevent disruption of activated switching sites during activation of other switching sites.